Add BackupAgent.onRestoreFinished() callback

The agent's onRestoreFinished() method is called after all available
data has been delivered to the app, whether via the key/value restore
API or the full-data file-at-a-time API.  This gives the app a stable
opportunity to do any postprocessing that might be appropriate.

Also fixes a lingering bug in the framework's handling of backup
agent lifetimes.  In cases where an existing agent instances was
being rebound, the framework was forgetting to notify the dependent
that the agent was available.  This was causing timeouts and restore
failure.

Bug 16241004

Change-Id: I3f52b299312d30d38b0cba63a2cfaeb934991ef2